Wouldn't an AI that specializes in making other AI be an AGI if they can cooperate?
If the AI can indeed assess arbitrary scenarios and come up with solutions to handle them, then it would indeed be an AGI.


  What’s the constraint ?


It doesn't exist.  Current programmers are very good at developing AI that can handle specific tasks ("narrow AIs"), but it is currently impossible to build an AI that can assess and handle "general" situations (unlike your proposed algorithm, which possess that capacity).

Theoretically, we can have a program that can build other programs (genetic algorithms are arguably one such example), but handling arbitrary scenarios and problems requires a form of "general intelligence", which we don't know how to program. Therefore, we can't build this machine.

It's possible that we can built this machine, but we must first figure out  the hard problem of "general intelligence". We're nowhere near reaching that level.

If we figure out how to program "general intelligence", then it should be fairly simple to use your approach (building an AGI to assess scenarios and then build "narrow AIs" that can handle Those scenarios). Only then we can understand the AGI's limitations and weaknesses, and be able to identify probable constraints to its power. For example, it's possible that such an AGI may be slow in handling arbitrary scenarios and developing the "narrow AIs"...in which case, it may take an absurdly long period of time to develop "a suite of AIs powerful/specialized for their tasks".

But until we build the AGI itself, we won't be able to identify its faults or weaknesses. Going beyond that would be science-fiction speculation.